WRL not scaling correctly on import
After downloading your file this is what I found ...


Your model surface normals are flipped randomly.
The flipped normal caused the add-on to give an incorrect readout.
To solve the issue go to Shading/Uvs tab on your 3dview Tool Panel here  


  


Click on the Recalculate button in the Normals section.
Now your Normals are rearranged so they are facing the right side up.
Do the add-on volume calculation will give you the correct result.


REMOVE OPEN EDGES...
1. Go into edit mode press Tab
2. Make sure to select all your model's faces.
3. Hit W and select remove double.
4. Remove double command here :